[in_app_purchase] update min Flutter version to 1.20.0  (#3887)

diff --git a/packages/in_app_purchase/in_app_purchase/CHANGELOG.md b/packages/in_app_purchase/in_app_purchase/CHANGELOG.md
index 5e369b8..022e39e 100644
--- a/packages/in_app_purchase/in_app_purchase/CHANGELOG.md
+++ b/packages/in_app_purchase/in_app_purchase/CHANGELOG.md
@@ -26,9 +26,10 @@
   // Present the code redemption sheet.
   await iosAddition.presentCodeRedemptionSheet();
   // Refresh purchase verification data.
-  PurchaseVerificationData? verificationData = await iosAddition.refreshPurchaseVerificationData(); 
+  PurchaseVerificationData? verificationData = await iosAddition.refreshPurchaseVerificationData();
   ```
   * The [store_kit_wrappers](https://pub.dev/documentation/in_app_purchase_ios/latest/store_kit_wrappers/store_kit_wrappers-library.html) have been moved into the [in_app_purchase_ios](https://pub.dev/packages/in_app_purchase_ios) package. They are still available in the [in_app_purchase](https://pub.dev/packages/in_app_purchase) plugin, but to use them it is necessary to import the correct package when using them: `import 'package:in_app_purchase_ios/store_kit_wrappers.dart';`;
+  * Update the minimum supported Flutter version to 1.20.0.
 
 ## 0.5.2
 
diff --git a/packages/in_app_purchase/in_app_purchase/example/pubspec.yaml b/packages/in_app_purchase/in_app_purchase/example/pubspec.yaml
index da9cf63..cd3086a 100644
--- a/packages/in_app_purchase/in_app_purchase/example/pubspec.yaml
+++ b/packages/in_app_purchase/in_app_purchase/example/pubspec.yaml
@@ -28,4 +28,4 @@
 
 environment:
   sdk: ">=2.12.0 <3.0.0"
-  flutter: ">=1.9.1+hotfix.2"
+  flutter: ">=1.20.0"
diff --git a/packages/in_app_purchase/in_app_purchase/pubspec.yaml b/packages/in_app_purchase/in_app_purchase/pubspec.yaml
index 4afceef..a91ff26 100644
--- a/packages/in_app_purchase/in_app_purchase/pubspec.yaml
+++ b/packages/in_app_purchase/in_app_purchase/pubspec.yaml
@@ -6,10 +6,10 @@
 dependencies:
   flutter:
     sdk: flutter
-  
+
   in_app_purchase_platform_interface: ^1.0.0
   in_app_purchase_android: ^0.1.0
-  in_app_purchase_ios: ^0.1.0 
+  in_app_purchase_ios: ^0.1.0
 
 
 dev_dependencies:
@@ -34,4 +34,4 @@
 
 environment:
   sdk: ">=2.12.0 <3.0.0"
-  flutter: ">=1.12.13+hotfix.5"
+  flutter: ">=1.20.0"
diff --git a/packages/in_app_purchase/in_app_purchase_ios/pubspec.yaml b/packages/in_app_purchase/in_app_purchase_ios/pubspec.yaml
index 49a63fc..fcc7c51 100644
--- a/packages/in_app_purchase/in_app_purchase_ios/pubspec.yaml
+++ b/packages/in_app_purchase/in_app_purchase_ios/pubspec.yaml
@@ -11,7 +11,7 @@
 
 dependencies:
   in_app_purchase_platform_interface: ^1.0.0
-  
+
   flutter:
     sdk: flutter
 
@@ -19,7 +19,7 @@
   collection: ^1.15.0
   meta: ^1.3.0
   test: ^1.16.0
-  
+
 dev_dependencies:
   build_runner: ^1.11.1
   json_serializable: ^4.1.1